Electroluminescent element

ABSTRACT

An electroluminescent device comprising an anode, a cathode, and at least one electroluminescent organic layer interposed therebetween, wherein a luminescent material in the organic layer emits light upon application of a voltage between the anode and the cathode, is characterized in that a salt of an electron accepting dopant with a polyimide precursor and/or polyimide having oligo-aniline units on side chains is formed as an auxiliary carrier transporting layer between the anode and the organic layer, the polyimide precursor and/or polyimide being obtained from a diamine component containing at least 1 mol % of an oligo-aniline unit-bearing diaminobenzene derivative and a tetracarboxylic dianhydride or derivative thereof.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/JP00/08524 which has an Internationalfiling date of Dec. 1, 2000 which designated the United States ofAmerica, the entire contents of which are hereby incorporated byreference.

TECHNICAL FIELD

This invention relates to an electroluminescent device comprising atleast one electroluminescent organic layer including a light emittingmaterial layer interposed between an anode and a cathode, wherein thelight emitting material layer emits light upon application of a voltagebetween the anode and the cathode.

BACKGROUND ART

The electroluminescent phenomenon of organic material was observed onanthracene single crystals (J. Chem. Phys., 38 (1963), 2042).Thereafter, a relatively intense luminescent phenomenon was observedusing a solution electrode having high injection efficiency (Phys. Rev.Lett., 14 (1965), 229). Thereafter, active research works were made onorganic luminescent materials between conjugated organic host materialsand conjugated organic activators having a fused benzene ring (U.S. Pat.Nos. 3,172,862, 3,173,050, 3,710,167, J. Chem. Phys., 44 (1966), 2902,and J. Chem. Phys., 50 (1969), 4364). The organic luminescent materialslisted herein, however, suffer from the drawbacks of increased filmthickness and a high electric field needed to induce luminescence.

As one countermeasure, researches were made on thin-film devices usingevaporation technique and succeeded in lowering drive voltage. Suchdevices, however, failed to provide luminance at a practicallyacceptable level (Polymer, 24 (1983), 748, and Jpn. J. Appl. Phys., 25(1986), L773).

Recently, Eastman Kodak proposed a device in which a charge transportinglayer and a light emitting layer are formed between electrodes by anevaporation technique, accomplishing a high luminance at a low drivevoltage (Appl. Phys. Lett., 51 (1987), 913 and U.S. Pat. No. 4,356,429).Thereafter, research works were further activated, as by shifting tothree layer type devices in which carrier transporting and lightemitting functions are separated. From then onward, the study on organicelectroluminescent devices entered the practical stage (Jpn. J. Appl.Phys., 27 (1988), L269, L713).

However, there remains a serious problem of product lifetime asdemonstrated by a luminescent life which is 3,000 hours at the shortestand several ten thousands of hours at the longest when operated atseveral hundreds of candelas.

It was also found that the above-described devices are prone todelamination due to moisture adsorption and thermal degradation andsubstantially increase dark spots during long-term service. It isbelieved that such degradation is mainly caused by interfacialseparation between the inorganic electrode and the organic layer and thepotential barrier between the electrodes and the respective carriertransporting materials although these problems remain outstanding.

DISCLOSURE OF THE INVENTION

Therefore, an object of the invention is to provide an organicelectroluminescent device which is restrained from thermal degradationand has improved heat resistance and durability.

Making extensive investigations to attain the above object, theinventors have found that in an electroluminescent device comprising atleast one electroluminescent organic layer interposed between the anodeand the cathode, especially an electroluminescent device in which anorganic hole transporting layer and a light emitting material layer aresequentially deposited on an inorganic electrode (ITO electrode etc.)serving as the anode and the cathode is disposed thereon, improvedadhesion and durability are achieved by providing an auxiliary carriertransporting layer between the anode and the organic layer (especiallybetween the inorganic electrode and the organic hole transportinglayer), and forming the auxiliary carrier transporting layer from asoluble, electrically conductive compound or polymer in the form of asalt that the polyimide precursor and/or polyimide defined below formswith an electron accepting dopant.

Specifically, the invention provides an electroluminescent devicecomprising an anode, a cathode, and at least one electroluminescentorganic layer interposed therebetween, wherein a luminescent material inthe organic layer emits light upon application of a voltage between theanode and the cathode, characterized in that a salt of an electronaccepting dopant with a polyimide precursor and/or polyimide havingoligo-aniline units on side chains is formed as an auxiliary carriertransporting layer between the anode and the organic layer, thepolyimide precursor and/or polyimide being obtained from a diaminecomponent containing at least 1 mol % of an oligo-aniline unit-bearingdiaminobenzene derivative represented by the following general formula(1) and a tetracarboxylic dianhydride or derivative thereof.

Herein R¹ to R⁹ each are independently hydrogen, an alkyl group, analkoxy group, a sulfonate group, or a substituted or unsubstitutedcyclohexyl, biphenyl, bicyclohexyl or phenylcyclohexyl group, n is apositive number of at least 1, A is a single bond or a divalent organicgroup selected from the group consisting of —O—, —COO—, —CONH— and —NH—,and R⁰ is a trivalent organic group containing an aromatic ring.

According to the invention, an aniline oligomer is attached to thepolyimide backbone as a side chain, and the aniline oligomer is dopedwith a halogen, Lewis acid, protonic acid, transition metal compound orelectrolyte anion to impart electric conductivity for acquiring anelectrode function. This improves the adhesion between the inorganicelectrode and a hole transporting layer which is the organic layer whilemaintaining a hole transporting capability, and precludes interfacialphenomena such as separation for thereby improving the durability of thedevice itself.

BEST MODE FOR CARRYING OUT THE INVENTION

The electroluminescent device of the invention includes an anode, acathode and an electroluminescent organic layer sandwiched therebetween.

The anode and cathode used herein may be well-known electrodes. Forexample, the anode may be an inorganic electrode (or transparentelectrode) of ITO or the like formed on a glass substrate. The cathodemay be a metallic electrode of aluminum, MgAg alloy or the like.

The electroluminescent organic layer includes a light emitting materiallayer and may be of well-known construction. A laminate construction inwhich a hole transporting layer, a light emitting material layer, and acarrier transporting layer are sequentially stacked from the cathodeside is typical, though the invention is not limited thereto.

The hole transporting material is not critical although it is generallyselected from tertiary aromatic amines such as N,N,N-tris(p-toluyl)amine(TPD), 1,1-bis[(di-4-toluylamine)phenyl]cyclohexane,N,N′-diphenyl-N,N′-bis(3-methylphenyl)(1,1′-biphenyl)-4,4′-diamine,N,N,N′,N′-tetrakis(4-methylphenyl)(1,1′-biphenyl)-4,4′-diamine,N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-bisphenyl-4,4′-diamine, and4,4′,4″-tris(3-methylphenylamino)triphenyl-amine. Pyrazoline derivativesare also useful.

The carrier transporting material is not critical although generallyaromatic fused ring compounds and metal complex compounds are oftenused. Examples include metal complex compounds such astris(8-hydroxyquinoline)aluminum (Alq3) andbis(10-hydroxybenzo[h]quinolate)beryllium (BeBq2), 1,3,4-oxathiazolederivatives, 1,2,4-triazole derivatives, bis(benzimidazole) derivativesof perylene dicarboxyimide, and thiopyrane sulfone derivatives.

Examples of the light emitting material include metal complex compoundssuch as Alq3 and tris(5-cyano-8-hydroxyquinoline)aluminum (Al(Q-CN)),and dyes such as oxathiazoles, e.g.,biphenyl-p-(t-butyl)phenyl-1,3,4-oxathiazole, triazoles, allylenes, andcoumarins though is not limited thereto.

In the electroluminescent device of the invention, an auxiliary carriertransporting layer is interposed between the anode and the organiclayer, and when the organic layer includes a plurality of layers,between the anode and a layer disposed most closely thereto, typically ahole transporting layer, for assisting in charge transportation.

The auxiliary carrier transporting layer is a thin film of a salt of anelectron accepting dopant with a polyimide precursor and/or polyimidehaving oligo-aniline units on side chains, the polyimide precursorand/or polyimide being obtained from a diamine component containing atleast 1 mol % of an oligo-aniline unit-bearing diaminobenzene derivativerepresented by the following general formula (1) and a tetracarboxylicdianhydride or derivative thereof.

In formula (1), R¹ to R⁹ each are independently hydrogen, an alkylgroup, an alkoxy group, a sulfonate group, or a substituted orunsubstituted cyclohexyl, biphenyl, bicyclohexyl or phenylcyclohexylgroup, n is a positive number of at least 1, “A” is a single bond or adivalent organic group selected from among —O—, —COO—, —CONH— and —NH—,and R⁰ is a trivalent organic group containing an aromatic ring.

R¹ to R⁹ are most often hydrogen, although alkyl, alkoxy, cyclohexyl,biphenyl, bicyclohexyl, phenylcyclohexyl and sulfonate groups arepreferred for increasing solvent solubility. More preferably, R¹ to R⁹are hydrogen, alkyl groups having 1 to 20 carbon atoms or alkoxy groupshaving 1 to 20 carbon atoms. It is noted that R¹ to R⁹ may be the sameor different.

Examples of suitable alkyl groups include methyl, ethyl, propyl,isopropyl, butyl, t-butyl, hexyl, octyl, decyl, and dodecyl. Alkylgroups having 1 to 4 carbon atoms are especially preferred.

Examples of suitable alkoxy groups include methoxy, ethoxy, propoxy,isopropoxy, butoxy, isobutoxy, s-butoxy, t-butoxy, hexyloxy, octyloxy,decyloxy and dodecyloxy. Alkoxy groups having 1 to 4 carbon atoms areespecially preferred.

Illustratively, R⁰ is a trivalent organic group containing an aromaticring such as a phenyl, biphenyl or naphthyl group as represented below.

The diaminobenzene derivative used in the polyimide precursor and/orpolyimide according to the invention is composed of the diamine moiety,the oligo-aniline moiety and the linkage A joining them together asdescribed above. Although the synthesis method is not critical, thediaminobenzene derivative can be synthesized, for example, by the methoddescribed below.

For the synthesis of diamines, a general procedure is by firstsynthesizing a corresponding dinitro compound of the following generalformula (2) and then reducing the nitro group in a customary manner forconversion to an amino group.

Herein, R¹ to R⁹, R⁰, n and A are as defined above.

The method generally employed for the synthesis of the compound offormula (2) involves the steps of joining a dinitro moiety of thefollowing formula to an oligo-aniline moiety through the linkage A andthereafter, bonding substituents R¹ to R⁹, or the steps of previouslysynthesizing an oligo-aniline having substituents, and thereafter,joining a dinitro moiety thereto through the linkage A.

dinitro moiety:

The linkage “A” is a single bond, ether bond —O—, ester bond —COO—,amide bond —CONH— or secondary amine bond —NH—. These linking groups canbe formed by conventional organic synthesis techniques.

For example, the ether bond is generally formed by reacting acorresponding halide derivative with a hydroxyl-substituted derivativein the presence of an alkali. The ester bond is generally formed byreacting a corresponding acid chloride with a hydroxyl-substitutedderivative in the presence of an alkali. The amide bond is generallyformed by reacting a corresponding acid chloride with anamino-substituted derivative in the presence of an alkali. The secondaryamine bond is generally formed by effecting dehydration condensationreaction of a corresponding primary amine group with ahydroxyl-substituted derivative.

Examples of the reactant from which the dinitro moiety is formed includedinitrobenzene, dinitronaphthalene and dinitrobiphenyl which have beensubstituted with a substituent to form the linkage A, for example, ahalogen atom, hydroxyl, haloacyl or amino group. The preferred reactantis a dinitrobenzene having the substituent. Examples of the substituteddinitrobenzene include 2,3-dinitrobenzene, 2,4-dinitrobenzene,2,5-dinitrobenzene, 2,6-dinitrobenzene, 3,4-dinitrobenzene and3,5-dinitrobenzene. However, from the standpoints of availability of thereactant and reactivity during polyimide polymerization,2,4-dinitrochlorobenzene, 2,4-dinitrophenol, 2,4-dinitrobenzoic chlorideand 2,4-dinitroaniline are most commonly used.

On the other hand, the oligo-aniline is obtained by effectingdechlorination ammonium reaction of an aromatic amine hydrochloridehaving the above substituent with an aromatic amine. In theoligo-aniline moiety, n has a value of 1 or more, and desirably n has avalue of 2 or more for electrical conductivity, and also a value of 20or less for solvent solubility. More desirably, n has a value of 8 orless.

The diaminobenzene derivative of general formula (1) obtained by themethod described above is then subjected to polycondensation with atetracarboxylic acid or derivative thereof, such as tetracarboxylicacid, tetracarboxylic dihalide and tetracarboxylic dianhydride, wherebya polyimide having oligo-aniline units on side chains is synthesized.

In the practice of the invention, the tetracarboxylic acids andderivatives thereof are not critical. Preferred are alicyclictetracarboxylic acids, especially 1,2,3,4-cyclobutanetetracarboxylicdianhydride, heterocyclic tetracarboxylic acids, aromatic ringtetracarboxylic acids, fused ring tetracarboxylic acids, and derivativesthereof.

Examples of such acids include

aromatic tetracarboxylic acids and dianhydrides and dicarboxylic aciddiacid halides thereof, such as, for example, pyromellitic acid,2,3,6,7-naphthalenetetracarboxylic acid,1,2,5,6-naphthalenetetracarboxylic acid,1,4,5,8-naphthalenetetracarboxylic acid,3,3′,4,4′-biphenyltetracarboxylic acid, 2,3,3′,4-biphenyltetracarboxylicacid, bis(3,4-dicarboxyphenyl) ether,3,3′,4,4′-benzophenonetetracarboxylic acid,bis(3,4-dicarboxyphenyl)-sulfone, bis(3,4-dicarboxyphenyl)methane,2,2-bis(3,4-dicarboxyphenyl)propane,1,1,1,3,3,3-hexafluoro-2,2-bis(3,4-dicarboxyphenyl)propane,bis(3,4-dicarboxyphenyl)diphenyl-silane, 2,3,4,5-pyridinetetracarboxylic acid, and 2,6-bis(3,4-dicarboxyphenyl)pyridine;

alicyclic tetracarboxylic acids and dianhydrides and dicarboxylic aciddiacid halides thereof, such as, for example,1,2,3,4-cyclobutanetetracarboxylic acid,1,2,3,4-cyclopentanetetracarboxylic acid, 2,3,5-tricarboxycyclopentylacetic acid, and 3,4-dicarboxy-1,2,3,4-tetrahydro-1-naphthalene succinicacid; and

aliphatic tetracarboxylic acids and dianhydrides and dicarboxylic aciddiacid halides thereof, such as, for example,1,2,3,4-butanetetracarboxylic acid.

These tetracarboxylic acids and derivatives may be used alone or inadmixture of two or more.

According to the invention, by copolymerizing the tetracarboxylic acidor derivative thereof with the diaminobenzene derivative of generalformula (1), abbreviated hereinafter as diamine (1), and optionally,another ordinary diamine, abbreviated hereinafter as ordinary diamine,there is obtained a polyimide having a molecular chain having anelectrically conductive side chain, which is used as a coating. Thediamine used to produce the polyimide essentially includes diamine (1).

The ordinary diamine other than diamine (1) is selected from primarydiamines commonly used in the synthesis of polyimides and not critical.Examples of the ordinary diamine include aromatic diamines such asp-phenylenediamine, m-phenylenediamine, 2,5-diaminotoluene,2,6-diaminotoluene, 4,4′-diaminobiphenyl,3,3′-dimethyl-4,4′-diaminobiphenyl, diaminodiphenylmethane,diaminodiphenyl ether, 2,2′-diaminodiphenylpropane,bis(3,5-diethyl-4-aminophenyl)methane, diaminodiphenyl-sulfone,diaminobenzophenone, diaminonaphthalene, 1,4-bis(4-aminophenoxy)benzene,1,4-bis(4-aminophenyl)benzene, 9,10-bis(4-aminophenyl)anthracene,1,3-bis(4-aminophenoxy)-benzene,4,4′-bis(4-aminophenoxy)diphenylsulfone,2,2-bis[4-(4-aminophenoxy)phenyl]propane,2,2-bis(4-aminophenyl)hexafluoropropane, and2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane; alicyclic diaminessuch as bis(4-aminocyclohexyl)methane andbis(4-amino-3-methylcyclohexyl)methane; aliphatic diamines such astetramethylenediamine and hexamethylenediamine; and diaminosiloxanes ofthe formula shown below. These diamines may be used alone or inadmixture of two or more. It is noted that the polyimide can be improvedin surface properties such as water repellency by adjusting the ratio ofthe moles of diamine (1) to the total moles of diamines used duringpolyimide polymerization.

Herein, m is an integer of 1 to 10.

Once a polyimide precursor is obtained by reacting the tetracarboxylicacid or derivative thereof with the diamine (1) and optionally, theordinary diamine for polymerization, the polyimide precursor is thensubjected to ring-closing imidization. The tetracarboxylic acid orderivative thereof used herein is typically a tetracarboxylicdianhydride. The ratio of the moles of the tetracarboxylic dianhydrideto the total moles of the diamine (1) and the ordinary diamine combinedis preferably from 0.8 to 1.2. Like conventional polycondensationreaction, a polymer having a higher degree of polymerization is obtainedas this molar ratio becomes closer to 1.

Too low a degree of polymerization may result in a polyimide film havinginsufficient strength on use, whereas too high a degree ofpolymerization may worsen the efficiency of operation during polyimidefilm formation.

Therefore, the product of the above reaction should preferably have sucha degree of polymerization that a polyimide precursor solution (having aconcentration of 0.5 g/dl in N-methylpyrrolidone at a temperature of 30°C.) may have a reduced viscosity of 0.05 to 5.00 dl/g.

In reacting the tetracarboxylic dianhydride with the primary diamine forpolymerization, any desired method may be employed. One commonly usedmethod is by dissolving the primary diamine in a polar organic solventsuch as N-methylpyrrolidone, N,N-dimethylacetamide orN,N-dimethylformamide, adding the tetracarboxylic dianhydride to thesolution, and reacting them to synthesize a polyimide precursor,followed by dehydration ring-closure reaction for imidization.

When the tetracarboxylic dianhydride is reacted with the primary diamineto form the polyimide precursor, the reaction temperature may be anytemperature selected in the range of −20° C. to 150° C., preferably −5°C. to 100° C.

The polyimide precursor can be converted to a polyimide by heating thepolyimide precursor at 100° C. to 400° C. for dehydration or subjectingthe polyimide precursor to chemical imidization in the presence of acommonly used imidization catalyst such as triethylamine/aceticanhydride.

When the polyimide according to the invention is prepared, thediaminobenzene derivative of formula (1), simply diamine (1), is used inan amount of at least 1 mol %, and preferably at least 5 mol % of theentire diamines.

In forming a coating of the polyimide, most often the polyimideprecursor solution is directly applied to a substrate and heated on thesubstrate for imidization to form a polyimide coating. The polyimideprecursor solution used herein may be the polymerization solution assuch or a solution obtained by pouring the polyimide precursor formedinto a large volume of a poor solvent such as water or methanol,recovering the precipitate, and dissolving it again in a solvent.

The diluent solvent for the polyimide precursor and/or the solvent fordissolving again the precipitated and recovered polyimide precursor maybe any solvent as long as the polyimide precursor is dissolvabletherein.

Examples of the solvent include N-methylpyrrolidone,N,N-dimethylacetamide and N,N-dimethylformamide. These solvents may beused alone or in admixture. Even a solvent which by itself cannot form auniform medium may be added to the above solvent insofar as a uniformmedium is obtainable. Examples of such solvents include ethylcellosolve, butyl cellosolve, ethyl Carbitol, butyl Carbitol, ethylCarbitol acetate, and ethylene glycol.

In forming a polyimide coating on a substrate, it is, of course,preferred to add an additive such as a coupling agent to the polyimidesolution for the purpose of further enhancing the adhesion between thepolyimide coating and the substrate.

The temperature used for heat imidization may be any temperature in therange of 100 to 400° C., with a range of 150 to 350° C. being especiallypreferred.

On the other hand, if the polyimide according to the invention issoluble in a solvent, the polyimide precursor resulting from reaction ofthe tetracarboxylic dianhydride with the primary diamine may be imidizedin the solution to form a polyimide solution. When the polyimideprecursor in the solution is converted into a polyimide, a method ofinducing dehydration ring-closure by heating is generally employed. Thetemperature of ring-closure by heat dehydration is any temperatureselected in the range of 150 to 350° C., and preferably 120 to 250° C.

Another method of converting the polyimide precursor to the polyimide ischemical ring-closure using well-known dehydration ring-closurecatalysts. The polyimide solution thus obtained may be used withoutfurther treatment or after it is precipitated in a poor solvent such asmethanol or ethanol, isolated and dissolved again in a suitable solvent.The solvent used for dissolving the polyimide again is not critical aslong as the polyimide is dissolvable therein. Exemplary solvents include2-pyrrolidone, N-methylpyrrolidone, N-ethylpyrrolidone,N-vinylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, andγ-butyrolactone. Besides, a solvent which by itself cannot dissolve thepolyimide may be added to the foregoing solvent insofar as the desiredsolubility is not impaired. Examples of such solvents include ethylcellosolve, butyl cellosolve, ethyl Carbitol, butyl Carbitol, ethylCarbitol acetate, and ethylene glycol.

In forming a polyimide coating on a substrate, it is, of course,preferred to add an additive such as a coupling agent to the polyimidesolution for the purpose of further enhancing the adhesion between thepolyimide coating and the substrate.

By applying the solution onto a substrate and evaporating the solvent, apolyimide coating can be formed on the substrate. Any temperature may beused at this stage as long as the solvent evaporates. Usually atemperature of 80 to 150° C. is sufficient.

The coating method of forming a thin film of the polyimide according tothe invention includes dipping, spin coating, transfer printing, rollcoating and brush coating, but is not limited thereto. The coatingthickness is not critical although a coating as thin as possible isdesirable for improving external emission efficiency. Usually, athickness of 100 to 1,000 Å is preferred.

According to the invention, the polyimide precursor and/or polyimidehaving oligo-aniline units on side chains as described above forms asalt with an electron accepting dopant, which salt is interposed as anauxiliary carrier transporting layer between the anode and the organiclayer.

With respect to the salt formation or doping of the polyimide precursorand/or polyimide having oligo-aniline units on side chains with anelectron accepting dopant, the electron accepting dopant is selectedfrom among Lewis acids, protonic acids, transition metal compounds,electrolyte salts, halides and the like. Doping with these dopantsenables to form polyimide thin films with a lower resistance.

Lewis acids include FeCl₃, PF₅, AsF₅, SbF₅, BF₅, BCl₃, and BBr₃.

Protonic acids include inorganic acids such as HF, HCl, HNO₅, H₂SO₄ andHClO₄, and organic acids such as benzene sulfonic acid,p-toluenesulfonic acid, dodecylbenzenesulfonic acid, polyvinylsulfonicacid, methanesulfonic acid, 1-butanesulfonic acid, vinylphenylsulfonicacid and camphorsulfonic acid.

Transition metal compounds include FeOCl, TiCl₄, ZrCl₄, HfCl₄, NbF₅,NbCl₅, TaCl₅ and MoF₅.

Electrolyte salts include LiSbF₆, LiAsF₆, NaAsF₆, NaSbF₆, KAsF₆, KSbF₆,[(n-Bu)₄N]AsF₆, [(n-Bu)₄N]SbF₆, [(n-Et)₄N]AsF₆ and [(n-Et)₄N]SbF₆.

Halides includes Cl₂, Br₂, I₂, ICl, ICl₃, IBr and IF.

Of these electron accepting dopants, ferric chloride is the preferredLewis acid, hydrochloric acid is the preferred protonic acid, perchloricacid is the preferred inorganic acid, and p-toluenesulfonic acid andcamphorsulfonic acid are the preferred organic acids.

Any desired method may be used in doping the polyimide precursor orpolyimide with the above dopants. Doping may be carried out in asolution of the polyimide precursor or polyimide in an organic solventor after a thin film is formed therefrom.

When doping is carried out in a solution of the polyimide precursor orpolyimide in an organic solvent, ferrous chloride among the transitionmetal compounds and p-toluenesulfonic acid and other organic acids aredesirably used as the dopant. The doping concentration differs dependingon the molecular weight of the aniline oligomer. In general, the dopantis preferably added such that one or less dopant molecule is availableper nitrogen atom in the aniline oligomer. Alternatively, after acoating is formed, it can be doped by exposing it to hydrochloric acidvapor or iodine vapor.

In forming the auxiliary carrier transporting layer according to theinvention, when the polyimide precursor and/or polyimide is doped in itsorganic solvent solution, this solution is used to form a thin film by awell-known method on the inorganic electrode (transparent electrode) ofITO etc. which has been formed on the glass substrate and serves as theanode. The inorganic electrode used herein has been removed of foreignmatter such as organic matter on the surface by cleaning treatment suchas back sputtering, ozone treatment or acid pickling.

After the auxiliary carrier transporting layer is formed on theelectrode-bearing substrate in this way, organic layers forelectroluminescence are deposited. The layer structure largely variesand is not critical. Most often, a device in which a hole transportinglayer, a light emitting layer and a carrier transporting layer aresequentially deposited by evaporation is used.

As the hole transporting material, carrier transporting material andlight emitting material, the aforementioned compounds are used. Thesematerials are sequentially deposited by vacuum evaporation and on thetop of them, a MgAg alloy is evaporated as a cathode. This results in anelectroluminescent device which emits light of a specific wavelengthupon application of a voltage thereacross.

Examples are given below for illustrating the present invention althoughthe invention is not limited to the examples.

EXAMPLE 1

A polyimide precursor solution was prepared, as shown by the schemebelow, by dissolving 3 g (0.00786 mol) of[4-{(4-(2,4-diaminophenoxy)phenyl)amino}phenyl]phenylamine in 25.22 g ofN-methylpyrrolidone, adding 1.45 g (0.00741 mol) of4,9-dioxatricyclo[5.3.0.0^(2,6)]decane thereto, and effectingpolycondensation reaction for 24 hours at room temperature.

The polyimide precursor thus obtained had a reduced viscosity of 0.52dl/g (0.5 wt %, 25° C.). This solution was coated onto a glass substrateand heat treated at 250° C. for one hour, forming a uniform polyimidefilm. IR analysis confirmed that the coating was a polyimide containinganiline oligomers.

After the polyimide varnish obtained above was doped withcamphorsulfonic acid as the dopant, it was applied onto an ITO electrodeto a thickness of 100 Å. Thereafter, TPD of 400 Å thick, Alq of 600 Åthick, and MgAg were successively deposited thereon by an evaporationtechnique.

The thus fabricated device was measured for threshold voltage foremission, maximum luminance, luminous efficiency and current efficiency.For a similar sample using 5-sulfosalicylic acid as the dopant, the sameproperties were measured. The results are shown in Table 1.

TABLE 1

Electroluminescent device's properties camphorsulfonic 5-sulfosalicylicDopant acid acid Threshold voltage for 9 9 emission (V) Maximumluminance (cd/m²) 3200 (25 V) 1300 (20 V) Luminous efficiency (lm/W)0.81 (17 V) 0.49 (24 V) Current efficiency (cd/A) 4.73 (20 V) 3.54 (24V)

EXAMPLE 2

A polyimide precursor solution was prepared, as shown by the schemebelow, by dissolving 3 g (0.00640 mol) of[4-{(4-(2,4-diaminophenoxy)phenyl)amino}phenyl](4-phenyl-amino)phenylaminein 25.22 g of N-methylpyrrolidone, adding 1.45 g (0.00741 mol) of4,9-dioxatricyclo[5.3.0.0^(2,6)]decane thereto, and effectingpolycondensation reaction for 24 hours at room temperature.

The polyimide precursor thus obtained had a reduced viscosity of 0.55dl/g (0.5 wt %, 25° C.). This solution was coated onto a glass substrateand heat treated at 250° C. for one hour, forming a uniform polyimidefilm. IR analysis confirmed that the coating was a polyimide containinganiline oligomers.

After the polyimide varnish obtained above was doped withcamphorsulfonic acid as the dopant, it was applied onto an ITO electrodeto a thickness of 100 Å. Thereafter, TPD of 400 Å thick, Alq of 600 Åthick, and MgAg were successively deposited thereon by an evaporationtechnique.

The thus fabricated device was measured for threshold voltage foremission, maximum luminance, luminous efficiency and current efficiency.For a similar sample using 5-sulfosalicylic acid as the dopant, the sameproperties were measured. The results are shown in Table 2.

TABLE 2

Electroluminescent device's properties camphorsulfonic 5-sulfosalicylicDopant acid acid Threshold voltage for 6 6 emission (V) Maximumluminance (cd/m²) 2900 (21 V) 3700 (20 V) Luminous efficiency (lm/W)0.91 (13 V) 0.93 (15 V) Current efficiency (cd/A) 4.21 (16 V) 4.85 (17V)

The diaminobenzene derivatives used herein are easy to synthesize andare used as one of reactants to produce polyimides having improved heatresistance, film strength and coating properties as well as antistaticand low charge accumulation properties. Using the polyimides as theauxiliary carrier transporting layer, electroluminescent devices havingenhanced reliability are obtainable.

What is claimed is:
 1. An electroluminescent device comprising an anodea cathode; at least one electroluminescent organic layer interposedtherebetween; and an auxiliary carrier transporting layer between theanode and organic layer, wherein said at least one electroluminescentorganic layer comprises a luminescent material that emits light uponapplication of a voltage between the anode and the cathode; saidauxiliary carrier transporting layer is formed from a salt of anelectron-accepting dopant and a polyimide precursor and/or polyimidehaving oligo-aniline units on side chains, and the polyimide precursorand/or polyimide being obtained from a diamine component containing atleast 1 mol % of an oligo-aniline unit-bearing diaminobenzene derivativerepresented by the following general formula (1) and a tetracarboxylicdianhydride or derivative thereof,

wherein R¹ to R⁹ are independently selected from the group consisting ofhydrogen, an alkyl group, an alkoxy group, a sulfonate group, and asubstituted or unsubstituted cyclohexyl, biphenyl, bicyclohexyl orphenylcyclohexyl group, n is a positive number of at least 1, A is asingle bond or a divalent organic group selected from the groupconsisting of —O—, —COO—, —CONH— and —NH—, and R⁰ is a trivalent organicgroup containing an aromatic ring.
 2. The electroluminescent device ofclaim 1 wherein in formula (1), R¹ to R⁹ each are independentlyhydrogen, an alkyl group having 1 to 20 carbon atoms or an alkoxy grouphaving 1 to 20 carbon atoms, and n is an integer of 1 to
 20. 3. Theelectroluminescent device of claim 1 wherein the polyimide contains atleast 5 mol % of units based on the diaminobenzene derivative of formula(1).
 4. The electroluminescent device of claim 1 wherein thetetracarboxylic dianhydride or derivative thereof is an alicyclictetracarboxylic dianhydride or derivative thereof.
 5. Theelectroluminescent device of claim 4 wherein the alicyclictetracarboxylic dianhydride or derivative thereof is1,2,3,4-cyclobutanetetracarboxylic dianhydride.
 6. Theelectroluminescent device of claim 1 wherein the tetracarboxylicdianhydride or derivative thereof is a heterocyclic tetracarboxylicdianhydride or derivative thereof.
 7. The electroluminescent device ofclaim 1 wherein the tetracarboxylic dianhydride or derivative thereof isan aromatic ring tetracarboxylic dianhydride or derivative thereof. 8.The electroluminescent device of claim 1 wherein the tetracarboxylicdianhydride or derivative thereof is a fused ring tetracarboxylicdianhydride or derivative thereof.
 9. The electroluminescent device ofclaim 1 wherein the dopant is selected from the group consisting of aLewis acid, protonic acid, transition metal compound, electrolyte saltand halide.